If you value Skyrim's existence, you would join the Imperial Legion.
Do you think the Empire wanted to sign the white-gold concordat? No, they had to because the Aldmeri dominion was far too strong for them at the time. They signed it to let the Imperial Legion regain their strength so the Aldmeri can be taken on. it's those damn Stormcloaks that weaken the empire through this war, and do you think that Skyrim on it's own could handle a full on Thalmor war? No, not at all.